Docker 1 - Docker Introduction

Introduction

• The process of developing, deploying, and managing software applications can be complex.

• Development & Deployment Process:

  • Teams involved: Development team (creates software) and Operations team (deploys and manages infrastructure).
  • Example: Building an order-tracking portal used across company outlets.
  • Stages: Development, QA (Quality Assurance), Pre-production, and Production environments.

• Key Challenges:

  • Managing Hosting Environments:

    • Need for consistent software and hardware management across different environments.
    • Configuration of network access, data storage, and security must be consistent and reproducible.

  • Continuity in Software Delivery:

    • Consistent deployments are crucial.
    • Each deployment package should include everything needed for the application to function (e.g., packages, binaries, libraries).
    • Dependencies must match in terms of software versions and architecture.

  • Efficient Hardware Use:

    • Applications should run in isolation from others on the same hardware.
    • Aim to run multiple applications per server without interference, maximizing resource use.

  • Application Portability:

    • Portability is essential for scenarios like scaling or infrastructure failure.
    • The ability to redeploy software quickly to a new environment with minimal downtime, even if the infrastructure differs.

• Next Steps:

  • Before diving into Docker features that address these challenges, there will be a brief overview of Docker concepts and architecture.

What is Docker?

• Docker is a containerization platform used to develop, ship, and run containers.
• Docker doesn’t use a hypervisor, and you can run Docker on your desktop or laptop if you’re developing and testing applications.
• The desktop version of Docker supports Linux, Windows, and macOS.
• For production systems, Docker is available for server environments, including many variants of Linux and Microsoft Windows Server 2016 and above.
• Many clouds, including Azure, support Docker.

What is a container?

• A container is a loosely isolated environment that allows us to build and run software packages.
• These software packages include the code and all dependencies to run applications quickly and reliably on any computing environment.
• We call these packages container images.
• The container image becomes the unit we use to distribute our applications.

What is software containerization?

• Software containerization is an OS-virtualization method used to deploy and run containers without using a virtual machine (VM).
• Containers can run on physical hardware, in the cloud, on VMs, and across multiple operating systems.

Docker Architecture

• The Docker platform consists of several components that we use to build, run, and manage our containerized applications.

Docker Engine

• The Docker Engine is a client-server application with the following components:

  • Docker Daemon: A background service that runs on the host system. The Docker Daemon manages Docker objects, such as images, containers, networks, and volumes.
  • REST API: An interface that specifies how the Docker Daemon can interact with it.
  • Docker CLI (Docker Client): A command-line interface that allows users to interact with the Docker Daemon.
  • Docker Compose (Docker Server): A tool for defining and running multi-container Docker applications.

• The client communicates with the server using a REST API, which enables the client to also communicate with a remote server instance.

Docker Client:

• Two options:
  • Command-line interface (CLI) called docker.
  • Graphical User Interface (GUI) called Docker Desktop.
• Both interact with a Docker server using the Docker REST API to manage containers (local or remote).

Docker Server:

• A daemon named dockerd.
• Handles client requests via the Docker REST API.
• Manages the lifecycle of containers and can interact with other Docker daemons.

Docker Objects:

• Objects include networks, storage volumes, plugins, and other service items.
• These objects are created and configured to support container deployments.

Docker Hub:

• A SaaS-based Docker container registry.
• Default public registry for storing and distributing container images.
• Alternative options:
  • Private Docker registry.
  • Cloud provider options include Azure Container Registry for storing images for Azure services.

How Docker images work

• Docker Images Overview:

  • A container image is the unit used to distribute applications in Docker.
  • The container format is standardized for both developers and operations teams.

• Software Packaged in a Container:

  • Includes more than just the application code.
  • Comprises application code, system packages, binaries, libraries, configuration files, and the OS running in the container.
  • Example: A .NET Core MVC app deployed using nginx on Ubuntu Linux, all packaged into a single container image.

• What is a Container Image?

  • A container image is a portable package containing all the software needed to run an application.
  • When executed, a container image becomes a container (the in-memory instance).
  • Immutability: Once built, a container image cannot be changed. To make changes, a new image must be created.
  • This immutability ensures that the image used in production is identical to the one used in development and QA.

What is the host OS?

• The host OS is the operating system on which the Docker Engine is installed.
• Docker containers running on Linux share the host OS kernel, and don’t require a container OS as long as the binary can access the OS kernel directly.
• However, Windows containers need a container OS.
• The container depends on the OS kernel to manage services such as the file system, network management, process scheduling, and memory management.

What is the container OS?

• The container OS is the operating system that runs inside the container.
• We have the flexibility to include different versions of Linux or Windows operating systems in a container.
• This flexibility allows us to access specific OS features or install additional software our applications might use.
• The container OS is isolated from the host OS, and it’s the environment in which we deploy and run our application.
• Combined with the image’s immutability, this isolation means the environment for our application running in development is the same as in production.
• In our example, we’re using Ubuntu Linux as the container OS, and this OS doesn’t change from development or production.
• The image we use is always the same.

What is the Stackable Unification File System (UnionFS)?

• Unionfs (Stackable Unification File System):

  • Used to create Docker images by stacking directories (branches) so they appear merged, though physically separate.
  • Allows adding/removing branches while building the file system.

  • Layering in Docker Images:

    • Example: Building an image for a web app by layering Ubuntu as the base image, then adding nginx and the web app on top.
    • When a container runs, a final writable layer is created, which doesn’t persist after the container is destroyed.

• Base Image:

  • An image that uses the Docker scratch image (an empty image with no filesystem layer).
  • Assumes the application can directly use the host OS kernel.

• Parent Image:

  • A container image used as a starting point to create new images.
  • Example: Using an Ubuntu-based image that already has nginx installed to create a new image.
  • Typically includes a container OS.

• Difference Between Base and Parent Images:

  • Base Image: Offers more control over the final image contents; starts from scratch.
  • Parent Image: Builds on an existing image, usually with some OS or application components already included.
  • Note: On Windows, container images must be based on Windows base container images, provided and serviced by Microsoft.